The present invention relates generally to adaptive filters, and more particularly to control of values of tap coefficients in adaptive finite-impulse-response filters.
In an ideal digital communications system, at a transmitting node, symbols encoded with binary information are modulated onto signals that are transmitted across a communications network. At a receiving node, the signals are received, and the symbols are recovered and decoded. In practice, the source signals are degraded as they are transmitted through the communications network; noise is added along the signal path and at the receiver; and reflected signals can be generated at interfaces between various links and between various components. Recovery of the symbols, therefore, in general is not straightforward. In particular, intersymbol interference can cause incorrect recovery of the symbols, resulting in errors in the decoded binary information.
For digital transmission across analog circuits via voice-band modems, adaptive equalizers were developed to reduce the effects of signal degradation due to distortion. Adaptive equalizers can be implemented with adaptive finite-impulse-response (FIR) filters. In addition to adaptive equalization, adaptive FIR filters are used for a variety of applications, including echo cancellation, noise cancellation, linear prediction, and system identification. Adaptive FIR filters can be deployed in communications systems with a variety of transmission media, including electrical, optical, acoustic, radiofrequency, and microwave.
In an adaptive FIR filter, signals are processed by a series of taps; each tap has a corresponding tap coefficient. The tap coefficients are dynamically adjusted via an algorithm to minimize the least-mean-squares (LMS) error (or other function of error). In practice, the system is under-constrained, and multiple combinations of values of tap coefficients can yield approximately the same value of LMS error; consequently, values of tap coefficients can wander and grow. In some instances, a value of a tap coefficient can grow so large that a digital register holding the value of the tap coefficient overflows.
A method for controlling the tap coefficients was developed by Gitlin in 1982 for adaptive equalizers in voice-band modems [R. D. Gitlin et al., “The Tap-Leakage Algorithm: An Algorithm for the Stable Operation of a Digitally Implemented, Fractionally Spaced Adaptive Equalizer”, The Bell System Technical Journal, Volume 61, Number 8, October 1982, pages 1817-1839]. This method, referred to as tap leakage, is still currently the accepted solution for adaptive equalizers and other adaptive FIR filters. An improved method would be advantageous for various applications.